1. Field of the Invention
The present invention relates to a method of irradiating laser light, a laser irradiation system (including laser and an optical system for leading laser light output from the laser to an irradiated object (an object to be subjected to irradiation) for irradiating laser light, and a method of manufacturing a semiconductor device therewith.
2. Description of the Related Art
These days, wide research has been done on the technique of crystallizing an amorphous semiconductor film formed on an insulating substrate such as glass to form a semiconductor film with a crystalline structure (hereinafter, a crystalline semiconductor film). For crystallization, annealing such as thermal annealing with furnace annealing, rapid thermal annealing (RTA), or laser annealing has been examined. In crystallization, it is possible to employ one of these or to combine plural kinds thereof.
The crystalline semiconductor film has quite a high mobility, compared to the amorphous semiconductor film. Accordingly, the crystalline semiconductor film is used to form a thin film transistor (TFT), which is utilized, for example, for an active matrix liquid crystal display device that has a pixel portion including TFTs formed on a glass substrate or a pixel portion and a driving circuit both including TFTs formed on a glass substrate.
Usually, heat treatment at 600° C. or more for 10 hours or more is necessary in order to crystallize the amorphous semiconductor film with furnace annealing. Although it is quartz that is applicable to this crystallization as a material of the substrate, the quartz substrate is expensive, and especially, is really difficult to be processed into a large-sized substrate. It is given as a means for increasing a productive efficiency to make the substrate have a large size, and this is why research is done on the technique of forming the semiconductor film on the glass substrate that is inexpensive and easy to be process into a large-sized substrate. In recent years, it has been considered to use a glass substrate with a side over 1 m.
As an example of the research, thermal crystallization with a metal element has been developed, which makes it possible to lower a crystallizing temperature that used to be regarded as a problem. In the thermal crystallization, thermal treatment at 550° C. for 4 hours after adding a minute amount of element such as nickel, palladium, or zinc to the amorphous semiconductor film makes it possible to form the crystalline semiconductor film. Since the temperature of 550° C. is not higher than a deforming temperature of the glass substrate, there is no worry about deformation and the like (for example, Japanese Patent Laid-Open 7-183540)
In laser annealing, on the other hand, it is possible to give high energy to only the semiconductor film without increasing a temperature of the substrate too much. Accordingly, attention is paid to the laser annealing in terms of uses for not only the glass substrate with a low deforming temperature but also a plastic substrate and the like.
In an example of the laser annealing, pulse laser light represented by excimer laser is shaped in an optical system to become a square spot with several centimeters on a side or a linear shape with a length of 100 mm or more at an irradiated surface and the laser light is moved relatively with respect to an irradiated object to perform annealing. It is noted that the linear shape here does not mean a line strictly but means a rectangle (or an oblong) with a large aspect ratio. For example, the linear shape indicates a rectangle with an aspect ratio of two or more (preferably, 10 to 10000), which is included in laser light (rectangular shaped beam) that is rectangular in shape at the irradiated surface. The linear shape is necessary in order to secure an energy density for sufficient annealing to the irradiated object, and the laser light may have the rectangular shape or a planar shape providing that sufficient annealing can be performed to the irradiated object.
Thus manufactured crystalline semiconductor film has a plurality of crystal grains assembled, and the crystal grains have random positions and sizes. In order to manufacture a TFT in isolation on the glass substrate, the crystalline semiconductor film is divided into an island shaped pattern. In that case, it is too difficult to specify the position and the size of the crystal grain included in the island shaped pattern when the TFT is formed. Compared to an inside of the crystal grain, a boundary between the crystal grains (crystal grain boundary) has an amorphous structure and an infinite number of recombination centers and trapping centers existing due to crystal defects. It is known that when a carrier is trapped in the trapping center, potential of the crystal grain boundary increases to become a barrier against the carrier, and therefore, lowers a current transporting characteristic of the carrier. While the crystallinity of the semiconductor film of a channel forming region has an influence on characteristics of the TFT, it is almost impossible to form the channel forming region of a single-crystal semiconductor film by getting rid of the influence of the crystal grain boundary.
Recently, attention has been paid to the technique of irradiating continuous wave (CW) laser light to a semiconductor film while scanning with the CW laser in one direction to form a single-crystal grain extending long in the direction. It is considered that it is possible, with the technique, to form a TFT that has almost no crystal grain boundary at least in a cannel direction thereof.
In the technique, however, the CW laser with a wavelength band absorbed into the semiconductor film sufficiently is used. In the case of using YAG laser, for example, conversion into higher harmonic is necessary. Accordingly, only laser that has a quite small output on the order of 10 W is applicable and the productivity is inferior compared to the case of using excimer laser. It is noted that appropriate CW laser for the technique has a high output, a wavelength of visible light or a shorter wavelength than visible light, and particularly high stability of the output, and laser such as second harmonic of YVO4 laser, second harmonic of YAG laser, second harmonic of YLF laser, second harmonic of YAlO3 laser, and Ar laser is applicable. Although the other harmonic can be used for annealing without problems, there is a disadvantage of a small output. When the recited laser above is used for annealing, however, irregularity in irradiation is likely to occur. In addition, the output is quite mall, which has trouble in term of throughput.